World energy consumption is predicted to increase 54% between 2001 and 2025. Considerable research effort is being directed towards the development of sustainable and carbon neutral energy sources to meet future needs.
Biofuels are an attractive alternative to current petroleum-based fuels, as they can be utilized in transportation with little change to current technologies and have significant potential to improve sustainability and reduce greenhouse gas emissions.
Biofuels include fuel ethanol. Fuel ethanol is produced from biomass by converting starch or other carbohydrates to sugars, fermenting the sugars to ethanol, and then distilling and dehydrating the ethanol to create a high-octane fuel that can substitute in whole or in part for gasoline.
In North America, the feedstock for the production of fuel ethanol is primarily corn, while in Brazil sugar cane is used. There are disadvantages to using potential food or feed plants to produce fuel. Moreover, the availability of such feedstocks is limited by the overall available area of suitable agricultural land. Therefore, efforts are being made to generate ethanol from non-food sources, such as cellulose, and from crops that do not require prime agricultural land.
One such non-food source is lignocellulosic biomass. Lignocellulosic biomass may be classified into four main categories: (1) wood residues (sawdust, bark or other), (2) municipal paper waste, (3) agricultural residues (including corn stover, corncobs and sugarcane bagasse), and (4) dedicated energy crops (which are mostly composed of fast growing tall, woody grasses such as switchgrass and miscanthus).
Lignocellulosic biomass is composed of three primary polymers that make up plant cell walls: Cellulose, hemicellulose, and lignin. Cellulose fibres are locked into a rigid structure of hemicellulose and lignin. Lignin and hemicelluloses form chemically linked complexes that bind water soluble hemicelluloses into a three dimensional array, cemented together by lignin. Lignin covers the cellulose microfibrils and protects them from enzymatic and chemical degradation. These polymers provide plant cell walls with strength and resistance to degradation, which makes lignocellulosic biomass a challenge to use as substrate for biofuel production.
There are two main approaches to the production of fuel ethanol from biomass: thermochemical and biochemical. Thermochemical processes convert the biomass to a reactive gas called syngas. Syngas is converted at high temperature and pressure to ethanol by a series of catalyzed processes. Biochemical processes use biocatalysts called enzymes to convert the cellulose and hemicellulose content to sugars, which are then fermented to ethanol and other fuels such as butanol.
Biochemical conversion of lignocellulosic biomass to ethanol in general involves five basic steps (1) Feed preparation—the target biomass is cleaned and adjusted for size and moisture content; (2) Pretreatment—exposure of the raw biomass to high pressure and temperature for a specified duration; with or without catalyzing additives; (3) Hydrolysis—conversion of the pretreated biomass to simple sugars using special enzyme preparations to hydrolyze pretreated plant cell-wall polysaccharides to a mixture of simple sugars; (4) Fermentation, mediated by bacteria or yeast, to convert these sugars to fuel such as ethanol; and (5) Distillation and Dehydration of the ethanol/fuel.
Efforts to ferment glucose (derived from cellulose) and xylose (derived from hemicellulose) simultaneously have been largely unsuccessful. The fermentation of the glucose component is easily carried out even in the harsh conditions following pretreatment and hydrolysis, however the fermentation of xylose has not been demonstrated on a commercial scale. Yeasts exist that will ferment xylose in pure sugar streams but they are ineffective in fermenting xylose in hydrolysates produced from lignocellulosic biomass, due to their sensitivity to inhibitory compounds in the hydrolysate.
In known pre-hydrolysis kraft dissolving pulp production process, the biomass wood chips are pre-treated in a batch pre-hydrolysis to remove hemicellulose. Water and chemical intensive batch washing systems, low recovery yield of soluble hemicellulose sugars and recovery of hemicellulose sugars primarily in oligomeric hence unfermentable form are the main defects of these processes.
Thus, compared to the prior art processes, a more economical and effective approach for dealing with the problem of xylo-oligomers and inhibitory compounds produced during pretreatment, is desirable.